General considerations - sample preparation if the latex is film forming

3 General considerations - sample preparation if the latex is film forming 

Unless a microscopic technique can be used, which allows the sample to remain in the liquid state, for example, optical microscopy (OM), colloidal probes in AFM, environmental or low vacuum scanning electron microscopy (Uwins, 1994), the preparation and analysis of film forming dispersions present a number of problems, where the size, microstructure and composition shall remain imchanged throughout the treatment. The sample preparation is often tedious and difficult to perform but there are useful techniques that in most cases give reliable results (Shaffer etal, 1987). 

One common problem arising when AFM experiments are run in air is that there might be a 2-50 nm thick contamination layer on the sample surface, which usually includes water that also can condense in the gap between the tip and the surface, even if it is absent elsewhere. If a contamination layer exists, capillary forces will pull the tip toward the surface with a strength that can be greater than the van der Waals forces, and will depend on the sample, the humidity and the tip shape. Contamination can be observed as hysteresis in the force distance curve since the tip will remain wetted up to larger separations as it is withdrawn from the surface but can be avoided by operating in vacuum, in dry gas or in a liquid. 

In contact mode the tip and sample remain in close contact as the scanning proceeds so that the deflecting force is repulsive. The specimen topography is measured by sliding the probe tip across the sample surface and the measurement is influenced by lateral forces. Soft samples can be damaged and may also result in distorted images. 

Tapping mode measures topography by tapping the sample surface with the cantilever near or at its resonance frequency with typical amplitudes between 20 and 100 nm (Zhong et al, 1993). Because the cantilever taps the surface for a very small fraction of its oscillation period it is prevented from being trapped by adhesive meniscus forces from the contaminant layer (Ktihle et al, 1997,1998 Knoll et al., 2001). Since the contact time 

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